I'm a designer for print and the huge problem with my new MBA is getting large files on to it from my Mac Pro... there is just not good way to do it since my Mac Pro doesn't have Thunderbolt and my MBA doesn't have Firewire... presenting a huge problem for me and rendering my MBA virtually useless.
Also if you're used to the resolution of a Cinema Display as I am, the MBA appears a tad low res... the screens on them just aren't that great.
That said, if these issues don't affect you, I have used the MBA occasionally for Photoshop work and it holds up pretty well... and if you're mobile you just can't beat the size and weight factor.
Hope this helps a little...
I'm not overly concerned with file transfers. Most, if not all, of the new files I'll be getting will be going over the network and, from what I've observed, most of the other Mac users appeared to have Thunderbolt Macs.
Speaking of networks, it should be possible for you to transfer the files over ethernet at a faster speed than FireWire anyway.
I'm sure, screen wise, a 13" laptop feels smaller than a Cinema Display, I have a 27" iMac I'm using at home so I'm used to larger screens. However, I also have an older 15" MBP and I hate carrying it around.
What sorts of files do you use in Photoshop? What file size and how complex? Do you do anything poster-sized and if so have you noticed any performance issues there?